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Archive 2015 · Focus issue - camera or lens?

  
 
shujert
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p.1 #1 · p.1 #1 · Focus issue - camera or lens?


I know this has been asked hundreds of times, but I'm having trouble finding an answer to my specific problem. The camera is a Leica M240, the lens in question is a CV 75/2.5 Color Heliar.

On the M, my 50mm M-Hex and 35mm Cron ASPH both focus accurately in both RF and LV. No issues that I can see.

The CV, however, front focuses pretty badly (6" or so) with the RF, but spot on in LV. Normally, I would chalk up the difference in RF vs LV accuracy to be a camera issue, but my other two lenses seem to be spot on using either focus mechanism.

Assuming an accurate RF, can a lens exhibit a difference using the RF vs LV? If so, why? Thanks in advance for the help.



Jul 24, 2015 at 10:28 AM
edwardkaraa
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p.1 #2 · p.1 #2 · Focus issue - camera or lens?


The lens rf coupling needs to be calibrated. Nothing unusual about it.


Jul 24, 2015 at 10:44 AM
shujert
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p.1 #3 · p.1 #3 · Focus issue - camera or lens?


Ah, got it, thank you. The lens cam strikes the focusing lever in the camera, which moves the prisms in the viewfinder. Live view takes the image straight from the sensor and projects it onto the LCD. Some basic reading on my part would have gone a long way


Jul 24, 2015 at 11:04 AM
rscheffler
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p.1 #4 · p.1 #4 · Focus issue - camera or lens?


I would suggest doing a test to check if the lens exhibits focus shift as it's stopped down. I don't have any experience with the CV75/2.5, however, I own the 75/1.8 and it front focuses wide open, but is about bang on at f/2.8, then back focuses slightly as it's stopped down until depth of field is deep enough. This is at nearer distances where focus shift is more problematic.

Some lenses with focus shift are intentionally RF optimized slightly stopped down because this balances the mis-focus to be less horrible in general use. One example was the ZM50/1.5, which IIRC, was originally optimized for f/2.8, resulting in front focus wide open. But now it's generally optimized for f/1.5 due to user demands of Zeiss wanting to primarily shoot it wide open...

My guess is my CV75/1.8 is f/2.8 optimized.



Jul 24, 2015 at 03:29 PM





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